In general, accurate measurement of a temperature of a substrate (e.g., a semiconductor wafer) to be processed by a substrate processing apparatus is very important in controlling the shape and property of a film, a hole and the like formed thereon by various processes such as film forming, etching and the like. For this reason, various methods of measuring a temperature of a semiconductor wafer, including a measurement method using a resistance thermometer or a fluorescent thermometer for measuring a temperature of a rear surface of a substrate and the like, have been performed.
In recent years, temperature measurement technology that uses a low-coherence interferometer and enables the direct measurement of the temperature of a wafer, which was difficult in the conventional temperature measurement method, is generally known. The temperature measurement technology using a low-coherence interferometer is configured such that light from a light source is split into measurement beam for temperature measurement and reference beam by a splitter, an interference between reflected measurement beam and the reference beam, which is reflected from a reference beam reflector provided with a driving mechanism for changing an optical path length, is measured, and temperature is measured (see, for example, Japanese Patent Application Publication No. 2006-112826).
In the above-described prior art, the temperature of a wafer can be directly measured through a simple construction. However, the inventors found as a result of detailed investigation that several types of noise are contained in a reference signal received from the reference beam reflector having the driving mechanism for changing an optical path length. For this reason, errors may be caused in detecting the center of gravity of an interference waveform, thus deteriorating the accuracies of an optical thickness measurement and a temperature measurement.